In the production of meat products, smoke treatment has a long tradition for taste and preservation reasons. In said tradition the smoking of the products in smoking rooms or chambers is widely used. However, in such a case the casings need to be permeable for the smoke, which means that the casing has a low water vapor barrier so that the meat product can lose weight during cooking and storage. To avoid weight loss during storage a secondary packaging may become necessary. However, smoke deposits on the walls and the ceiling of the smoking rooms can affect the appearance of meat products in the following cooking cycle. To avoid these drawbacks of a smoke treatment through a smoke-permeable casing, the inside of casings has been impregnated with liquid smoke, but also with colour solutions, e.g. solution on a caramel basis.
For a transfer of substances like liquid smoke or caramel solutions onto foodstuff, several casing structures have been described. As inner layers of the casing, some of these structures use hydrophilic materials, which allow the absorbance of the substance into the material. For example inner layers comprising starch were used. Other structures use as an inner layer a layer made of cellulose which will come into contact with the foodstuff. In such a case, however, absorption of the liquid into the paper or cellulose is observed. In case of a flat film, alternatively the desired substance can be sprayed, printed or scraped onto the surface. In order to achieve a sufficient fixation, a drying step has to be carried out before the flat film is formed into a tube.
The carrier film of these structures can be a fibrous layer, the major disadvantage of which is a low water vapor barrier effect. During a cooking process or storage of the product, the foodstuff loses part of its water content so as to reduce the yield. For this reason plastic coated fibrous casings and laminates comprising an outer plastic film and an inner paper or cellulose film have been developed. EP 0 992 194 A discloses a casing that consists of an impermeable film with a joined inner lining consisting of fibres made of cotton or cellulose, or woven, non-woven or knitted fabric, which is then impregnated and sealed to a tube or bag. Another example of a food casing which is capable of transferring food additives is described in US 2006/0003058 A, which teaches an at least two-layer tubular food casing comprising an inner layer formed of a thermoplastic organic polymer embedding a powdery organic filler such as starch. The casings described in these two documents combine the absorbance capacity of a material forming the inner layer with barrier properties provided by the outer (multi-layer) plastic films. However, such casings are disadvantageous because of a multi-step production process for preparing such coated or laminated casing structures, a possible mould growth on the inner starch, cellulose or paper layer and breaking problems during cooking.
U.S. Pat. No. 7,615,270 B2 discloses casings comprising an inner layer made of hydrophilic materials, such as e.g. block-copolyether ester or block-copolyether amide. Such casings suffer from the disadvantage that their absorbance capacity for liquid substances is limited. In many applications, such an absorbance capacity is insufficient in order to provide the desired effect onto the foodstuff. Furthermore, if liquid substance remains on the casing surface, a non-uniform distribution and formation of droplets upon opening of the tubular casing is likely and the transfer onto the foodstuff shows a marble-like, non-uniform coloration of the foodstuff.
DE 101 24 581 A teaches a food casing wherein liquid smoke is sprayed into the casing during shirring. As it takes a storage time of at least 5 days for the liquid smoke to migrate into the casing, the storage costs and the “time-to-customer” is high.
In general, the outer side of a casing is easily accessible for coating and impregnation. But, in the case of barrier films the casing needs to be turned inside out so that the treated surface will come into contact with the foodstuff. EP 1 192 864 A teaches a step of coating or impregnating the outer side of a casing which is stuffed into the bore of the strand so that it can be turned inside out during stuffing. Thus prior to a stuffing of the strand the treated outer surface of the casing can be contaminated during handling processes. When the treatment (coating/impregnation) is carried out by using a printing process, the edges are printed twice, which leaves two longitudinal lines of higher intensity on the foodstuff after peeling off the casing.
In order to increase the absorbance capacity of the thermoplastic inner layer, a formation of pores in that inner layer is desirable. Inner layers formed of a thermoplastic material as disclosed in EP 1 164 856 B1 show a network of interconnected interstices, which are in a range of 0.002 to 1 μm. Said inner layers are made from a food grade thermoplastic with the interstices being formed by a non-supercritical liquid pore-forming agent selected from soybean oil, peanut oil, corn oil, glycerine, sorbitol, polyethylene glycol, mineral oil or surfactants including polysorbate, polyoxyethylene (POE) 20, sorbitan monostearate, sorbitan monolaurate, sorbitan monooleate and glycerol monooleate. The thermoplastic material may further comprise an inorganic filler selected from silica (SiO2), talc (Mg2SiO4), aluminium oxide, hydrated alumina, titanium oxide, zirconium oxide, sodium silicate, silicate, sodium chloride, calcium, calcium carbonate, clay or calcined clay.
EP 1 911 352 A1 discloses a multi-layered coextruded stretched thermoplastic food casing comprising at least one porous inner layer, wherein the porosity of the porous inner layer has been at least partially generated by stretching the coextruded casing, wherein at least an innermost porous inner layer has an interconnected porosity, such that said innermost porous inner layer is able to absorb, retain, desorb and to transfer at least one transferable functional additive from said at least one porous inner layer to food encased in said casing. The casing further comprises at least one layer having a barrier effect for water vapor, at least one layer having adhesion properties, said layer having adhesion properties can be the same or different from said porous inner layer and/or said layer having a barrier effect for water. The porous inner layer comprises at least one plastic material and at least one fine-grained filler and at least one pore-forming component of an oily pore-forming agent and an emulsifying agent.
US 2009/214722 A1 discloses a multi-layered seamless tubular casing capable of absorbing, storing and releasing a food additive, comprising an outer layer of a thermoplastic polymer, at least one intermediate functional layer made of thermoplastic polymer with barrier properties against oxygen and/or water vapor and one porous inner layer made of a thermoplastic polymer with a network of spaces connected to one another, which are formed by using a non-supercritical liquid pore-forming agent bringing about a phase separation of the thermoplastic and the porosity modifier. The non-supercritical liquid pore-forming agent is selected from the group consisting of soybean oils, peanut oils, corn oils, glycerols, sorbitols, polyethylene glycols, mineral oils or surfactants, polysorbates, polyoxyethylene (POE) 20, sorbitan monostearates, sorbitan monolaurates, sorbitan monooleates, glycerol monooleates, Surf actol 365, or mixtures of the aforementioned substances. The inner layer may further comprise an inorganic filler selected from the group of silicon dioxide, talc (Mg2SiO4), aluminium oxide, aluminium hydroxide, hydrated alumina, calcined alumina, titanium oxide, zirconium oxide, sodium silicate and silicate.
The inner layer of the casings described in EP 1 911 35 2A1 and US 2009/214722 A1 requires a complex recipe and compounding step. The compounds of the examples described in EP 1 911 352 A1 comprise a relatively high percentage of low molecular liquid pore-forming agents (19% oil and 8% glycerine), which make a compounding challenging and cost-intensive. In US 2009/214722 A1 the recipe of the inner layer is not disclosed, but the reported absorbance capacities are low. Furthermore, all such casings suffer from the fact that the liquid pore-forming agents used at least partially remain in the inner layer.
Therefore, the object to be solved by the present invention is to provide a multilayered coextruded thermoplastic food casing having a high strength and improved peelability the inner layer(s) of which has/have a high porosity, high functional additive absorption capacity, excellent capability of transferring food additives to the enclosed foodstuff, wherein said inner layer(s) do(es) not comprise any liquid pore-forming substance, especially any non-supercritical liquid pore-forming agent after formation of the inner layer.
Furthermore, it is an object of the present invention to provide a process for preparing such an improved multi-layered coextruded food casing.